This patent pertains to the design and manufacture of linear and rotary synchronous motors and generators. The invention has application to virtually all synchronous machines, particularly those that use permanent magnets for the field.
The design and manufacture of synchronous motors has been carried on for over 100 years, but the applications have been primarily for low power motors where synchronism is essential and for high power motors where efficiency and size advantages overcome other disadvantages.
Nicola Tesla invented the induction motor in 1888 and since then the induction motor, also called an asynchronous motor, has dominated the market for rotary motors. The induction motor has been the preferred choice for electric utility powered motors because they have been less expensive to build and to control, and they were not cost-effective for most variable speed applications.
The increased use of synchronous motors is due to the evolution of microprocessors designed specifically for motor control, the availability of more cost effective power electronics, better permanent magnets, and the increased recognition that variable speed operation allows performance advantages. For example, the permanent magnet rotary synchronous motor is receiving serious attention as a candidate for electric vehicle propulsion and for low speed, high torque applications where induction motor alternatives are heavy, inefficient and expensive. The linear version of this motor, called the Linear Synchronous Motor (LSM), is now used for applications that cannot be effectively addressed by the linear induction motor (LIM) and as a replacement for rotary motors driving wheels.
It should be recognized that that there is usually very little difference between the design of a motor and the design of a generator (or alternator). Hence, in all discussion herein, the term motor should be interpreted as implying both motors and generators and machines designed to perform both functions.
There are several problems that motor designers must address: reducing size and weight, increasing efficiency, decreasing cost, eliminating cogging or ripple force, and simplifying manufacturing. Many patents have been issued that attempt to show how to improve synchronous machines with respect to these factors: U.S. Pat. No. 6,081,058 (Motor Structure Having a Permanent Magnet Motor with Grooves to Reduce Torque Ripples); U.S. Pat. No. 5,990,592 (Magnets Containing—Type Alternating—Current Motor and Method of Designing the Same); U.S. Pat. No. 5,757,100 (Method & Apparatus for Reducing Cogging Torque in an Electric Motor); U.S. Pat. No. 5,723,917 (Flat Linear Motor); U.S. Pat. No. 5,523,637 (Permanent Magnet Electrical Machine with Low Reluctance Torque); U.S. Pat. No. 5,521,451 and U.S. Pat. No. 5,126,606 (Low-cost Stepping or Synchronous Motor and Electric Drive Motor, Especially for Control and Regulation); U.S. Pat. No. 5,519,266 (High Efficiency Linear Motor); U.S. Pat. No. 5,444,341 (Method and Apparatus for Torque Ripple Compensation); U.S. Pat. Nos. 5,214,323 and 5,032,746 (Linear Motor with Reduced Cogging and Linear Motor with Driving Device). In every one of these patents, the focus is on solving a single problem and the result is degradation in some other aspect of the motor performance and/or manufacturing cost.
The problems identified above are so important that they are dealt with in some detail, and without adequate resolution, in reference books on synchronous motor design. Examples include Gieras, Jacek and Piech, Zbigniew, Linear Synchronous Motors, Transportation and Automation Systems, CRC Press, 2000; Nassar, Linear Motors, Wiley, 1994; Hughes, Austin; Electric Motors and Drives: Fundamentals, Types and Applications, 2nd Edition; Butterworth-Heinemann, 1993
In view of the foregoing, an object of the invention is to provide improved synchronous machines, including synchronous motors, and methods for design and manufacture thereof, with improved performance and lower manufacturing cost.